1. Field of the Invention
The present invention relates to non-oriented electrical steel sheets and a method for producing non-oriented steel sheets, and more particularly to compositions of the non-oriented electrical steel sheets and the terms of hot-rolling thereof.
2. Description of the Prior Arts
Non-oriented electrical steel sheets are widely used for core materials of electrical apparatus for example, a rotating machine. Recently, for increasing efficiency of, lightening and compacting these electrical apparatuses, materials having low core loss and high magnetic flux density have been in demand.
Steel sheets to which silicon is added, so-called silicon steel sheets have been customarily used as non-oriented electrical steel sheets. The addition of Si to steel increases specific resistance and reduces core loss value. However, because Si is an element having characteristic of allowing .alpha.-phase to be stabilized as shown in FIG. 1, Ar.sub.3 transformation point temperature of silicon steel is raised in compliance with addition of Si, and .gamma.-phase of the silicon steel closes its loop when the addition of Si reaches a certain amount. The .gamma.-phase of extra low carbon steel which contains no Al closes its loop at approximately 1.7 wt % Si while the critical Si-amount is decreased when Al is added to the extra-low carbon steel. Changes of Ar.sub.3 transformation point temperatures of such range of 800.degree. to 1,000.degree. meet finishing temperatures at hot rolling. Therefore, hot rolling in the whole length at the Ar.sub.3 transformation temperature becomes more difficult as Si addition amount is increased. That is to say, in the case of a steel containing 1.7 wt % Si as shown in FIG. 1, Ar.sub.3 transformation point temperature reaches 900.degree. C. and more. For this reason, conventional, methods do not permit finishing hot rolling temperatures above their Ar.sub.3 transformation points.
To overcome the difficulty the art has been forced to adopt high temperature heating. However, the means for heating Si contained steel sheets at high temperature of 1,200 .degree. C. and more has a disadvantage in that surface smoothness property of the Si contained steel sheets is deteriorated. This is because, when the silicon contained steel sheets are heated at high temperature of 1,200 .degree. C. and more, slab surface scales are melted, exfoliative features of the slab surface scales before hot rolling are lowered, and scales are rolled in during the process of hot rolling.
Moreover, even if the finishing temperature is maintained at Ar.sub.3 transformation point or more, by lower temperature heating, the means still has a drawback that magnetic property of the final products deteriorates, because, in this case, owing to edge portions of steel slabs being hot-rolled in the state of having ferrite and austenite dual phases, thickness and structure of the edge portions of hot-rolled steel sheets become nonuniform, due to difference of deformation resistance of the two phases.